xi ELECTRICAL FISHES 419 



good conductor, an intense current is developed in this, and by 

 opening the circuit at the appropriate moment, a gap is produced 

 at the instant of opening, which is smaller than can be attained 

 by placing two fixed metals in close proximity. Across such a 

 gap, the current, augmented by induction effects, will readily 

 pass as a spark. 



" If, on the other hand, there is a previous gap in the experi- 

 mental circuit, no matter how small, there will be no deriving 

 branch of current in the circuit, capable of sparking across it. 

 It is thus a delusion to suppose that the powerful discharge of 

 the electrical fish is incapable of bridging over the gap, for in 

 reality the gap prevents the development of the derived current, 

 which results in a powerful shock when well conducted. The 

 powerful shock that is expected to spark across the gap is 

 actually non-existent when the gap is present " (within certain 

 limits of extension for the gap). 



Accordingly, in all experiments on electrical fishes where 

 strength of effort is required, it is a rule to reduce the external 

 resistance in the leading -off circuit as much as possible. Du 

 Bois-Eeymoud first pointed out the adaptation of the different 

 electrical organs to the media in which they have to act. " The 

 organs of Torpedo require no great internal resistance in sea- 

 water, and can do with less E.M.F. ; they are short, with a wide 

 cross-section. The fresh- water organs of Malapterurus and G-ym- 

 notus 'require great internal resistance, ergo greater E.M.F. ; these 

 are long with a small section." 



Du Bois-Eeyniond first drew attention to the ease with which 

 a discharging spark can be elicited by the shock from a fish, with 

 the help of induction, by leading it through the primary coil of a 

 Euhmkorff 's inductorium. If a spark-micrometer is introduced into 

 the secondary circuit, two sparks will regularly appear, one larger 

 immediately followed by a smaller. Armand Moreau (du Bois- 

 Eeymond, 4 d, p. 628) even succeeded in showing the electroscopic 

 attraction and repulsion by the shock, on replacing the platinum 

 points of the spark-micrometer by two bent copper wires, to the 

 ends of which two gold leaves were attached. " At a distance of 

 3 mm. the movement of the leaves at the moment of the dis- 

 charge was doubtful, at 2 mm. they obviously attracted each 

 other, and at a less distance they flew together, with a magnificent 

 green flash, which left the leaves in cohesion." 



